Git flow using non-default branch names doesn't work

When I try to initialize git flow with a different branch name then develop, it still tries to use the branch develop.

I have a repo with a develop*ment* branch and a master branch. So I tell git flow to use development for the development branch. But it still tries to use develop.

After hitting OK in the initializing screen, it says:

git init -d 
Using default branch names.
Fatal: Local branch 'develop' does not exist.Which branch should be used for integration of the "next release"?
   - development
Branch name for "next release" development: [develop] 
Completed with errors, see above

I'm on macOS 10.15.5 (Catalina) and Sourcetree 4.0.1 (234).